Process for preparation of hollow fibers for fluid separator construction

ABSTRACT

A method for making hollow fibers having a selective permeability, wherein a majority of the hollow fibers each has on the periphery thereof 1 to 10 fins extending in the longitudinal direction, and the occupancy ratio y of the sections, defined by the peripheries, of all the hollow fibers exclusive of the fin portions in the hollow fiber bundle to the section, vertical to the axial direction of the hollow fiber bundle, of the inner wall of the shell is within a range defined by the following formula (I): 
     
         41-3.1√αx≦y≦61-3.1√αx (I) 
    
     wherein x stands for the average fin number per hollow fiber in the hollow fiber bundle, and α indicates the ratio H/d of the average fin height H (μ)in the hollow fiber bundle to the average outer diameter d (μ) of the hollow fibers exclusive of the fin portions.

This application is a continuation of application Ser. No. 235,845,filed Aug. 24, 1988, now abandoned which is a division of applicationSer. No. 796,865, filed Nov. 12, 1985, now U.S. Pat. No. 4,781,833.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fluid separator, a hollow fiber to beused-for construction thereof and a process for the preparation of thishollow fiber. More particularly, the present invention relates to ahollow fiber type fluid separator comprising hollow fibers having finson the periphery thereof, such a hollow fiber, and a process for thepreparation thereof.

As the fluid separation intended in the present invention, there can bementioned liquid separation such as dialysis, ultrafiltration, precisionfiltration, pervaporation, or reverse osmosis and gas separation such asoxygen enriching in air.

2. Description of the Related Art

Fluid separators comprising hollow fibers having a selectivepermeability have been practically used for the reverse osmosis orhemodialysis.

Especially, hollow fiber type blood dialyzers have been vigorously usedfor purifying blood of patients suffering from the renal insufficiency.In a blood dialyzer of this type, many permeable membranes, for example,hollow fiber membranes, are filled in a shell, and blood of a patient ispassed through the hollow interiors and a dialyzing solution (ordialysate) is passed outside the membranes, that is, spaces among themembranes. Wastes in the blood are removed through the hollow fibers bydialysis to correct the electrolyte concentration, and by producing adifference of the pressure between the inside and outside of the hollowfibers, excessive water is removed from the blood by ultrafiltration.Furthermore, hollow fibers are used for remedy of an autoimmune diseaseby separating only plasma from blood or removing a specific componentfrom the separated plasma. Hollow fibers to be used for such bloodtreatments should allow selective permeation of specific solutesaccording to the intended use. The capacity of a hollow fiber depends onthe material, the porosity (pore size and pore number) and the membranethickness of the hollow fiber For example, it is important how a numberof hollow fibers should be bundled so as to increase the dialyzingefficiency of the entire membrane surface. For example, when hollowfibers are arranged in the longitudinal direction closely to oneanother, the dialyzing solution does not flow uniformly around thehollow fibers but forms specific flow paths, with the result thatdialysis is hardly performed through hollow fibers not participating inthese flow paths and the entire dialyzing effect is reduced. In theordinary dialytic operation, the difference of the concentration betweenthe inside and outside of the hollow fiber membrane is a driving forcefor the transport of the solute. Accordingly, it is necessary to make acontrivance on the shape of the hollow fiber per se so that thedialyzing solution flows through the outside space of the hollow fiberas uniformly as possible, the area having a larger boundary layerresistance is reduced as much as possible and the difference of theconcentration between the blood side (the inner side of the hollowfiber) and the dialyzing solution side (the outer side of the hollowfiber) is increased.

As means for solving this problem, there has been proposed a method inwhich by increasing the amount contained (filling ratio) of hollowfibers in a dialyzer shell to some extent, a certain flow resistance isgiven to a dialyzing solution to uniformalize the flow thereof.Furthermore, there has been proposed a method in which hollow fibers arecrimped or cover yarns are wound around the hollow fibers to preventcontact of the hollow fibers with one another and increase theefficiency of the transport of the solute by the flowing of thedialyzing solution. However, these methods are still insufficient andfurther improvements are desired.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to solve theabove problem and provides a fluid separator valuable especially as ablood treating device having a high blood treatment efficiency.

Another object of the present invention is to provide a small-size fluidseparator having a high performance, in which finned hollow fibers thatcan be stably and easily prepared are contained in an optimum fillingstate.

Still another object of the present invention is to provide a finnedhollow fiber for use in this fluid separator and a process for thepreparation of this finned hollow fiber.

We made research with a view to attaining these objects and as theresult, it was found that in a fluid separator in which a bundle offinned hollow fibers extending in the longitudinal direction is set, ifthe shape of the hollow fibers and the filling state of the hollow fiberbundle are appropriately arranged, the separation efficiency is greatlyincreased. We have now completed the present invention based on thisfinding.

More specifically, in accordance with one fundamental aspect of thepresent invention, there is provided a fluid separator comprising ashell filled with a bundle of hollow fibers having a selectivepermeability, wherein a majority of the hollow fibers each has on theperiphery thereof 1 to 10 fins extending in the longitudinal direction,and the occupancy ratio y of the sections, defined by the peripheries,of all the hollow fibers exclusive of the fin portions in the hollowfiber bundle to the section, vertical to the axial direction of thehollow fiber bundle, of the inner wall of the shell is within a rangedefined by the following formula (I):

    41-3.1√ax≦y≦61-3.1√axα   (I)

wherein x stands for the average fin number per hollow fiber in thehollow fiber bundle, and α indicates the ratio H/d of the average finheight H (μ) in the hollow fiber bundle to the average outer diameter d(μ) of the hollow fibers exclusive of the fin portions

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 3, 4, and 5 are enlarged sectional views illustrating hollowfibers valuably used in the present invention.

FIGS. 6, 7, and 8 are diagrams illustrating examples of the shape of thenozzle for spinning hollow fibers valuably used in the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail. The mostcharacteristic feature of the present invention resides in the findingthat when finned hollow fibers are used, the number of the hollow fibersto be filled can be greatly reduced and if the configurations of thefinned hollow fibers, that is, the number and height of fins, and theamount of the hollow fibers to be filled in the shell satisfy specificrequirements, the dialysis performance per unit effective area of thehollow fibers is increased and the insertion of the hollow fibers intothe shell is greatly facilitated.

The term "sections defined by the peripheries" as used herein refers tothe sections of the radial thickness plus hollow core of the hollowfiber other than fin portions.

In the present invention, the number x of the fins in the hollow fiberis at least 1. However, if the number x of the fins is 11 or larger,reduction of the effective membrane area by the root portions of thefins becomes conspicuous, resulting in drastic reduction of the solutedialyzing performance and the water permeation performance. The number xis ordinarily 1 to 10, but it is preferred that the number x be 2 to 7,especially 3 to 6.

In the present invention, a is a parameter indicating the fin height.When a plurality of fins differing in the height are formed, thearithmetic mean value is used. In view of the separation efficiency, itis preferred that the value α be 0.01 to 0.8, especially 0.03 to 0.2.

The sections of typical examples of the finned hollow fiber valuablyused in the present invention are shown in FIGS. 1 through 5.

Other dimensions of the hollow fiber are not particularly critical inthe present invention However, it is preferred that the outer diameter dbe 100 to 400 μ, especially 200 to 300 μ, the membrane thickness h ofthe fin-free portion be 5 to 50 μ, especially 5 to 30 μ, particularlyespecially 10 to 25 μ, and the fin height H be 5 to 100 μ, particularlyespecially 9 to 60 μ. Moreover, it is preferred that the width W of theroot portion of the fin be narrower than the width of the upper portion.Ordinarily, after the spinning solution is extruded from a spinneret,the root portion tends to be expanded by the surface tension. It ispreferred that the width W be 15 to 50 μ, especially 20 to 40 μ. If thewidth is within this range, the circularity of the hollow fiberexclusive of the fin portion is good, and when the hollow fiber is usedfor a blood dialyzer, coagulation of blood in the hollow fiber is hardlycaused or blood is hardly left in the hollow fiber.

The sectional shape of the hollow fiber exclusive of the fin portion isnot limited to a circular shape, but the section may be ellipsoidal. Inthe ellipsoidal section, the outer diameter d is a mean value of thelong diameter and the short diameter. Furthermore, the fins on theperiphery of the hollow fiber may have a spiral shape. In the case wherea plurality of fins are formed on one hollow fiber, the respective finsmay be the same or different in the height H or the width W. At leasttwo hollow fibers may be connected through fins. In this case, however,the number of the connected hollow fibers is preferably 2.

In the fluid separator of the present invention, the shell preferablyhas a circular shape, and in this case, the above-mentioned occupancyratio y is expressed by the following formula (II):

    y=(Nd.sup.2 /D.sup.2)×100(%)                         (II)

wherein D stands for the inner diameter (μ) of the shell in which thehollow fiber bundle is filled, d stands for the average outer diameter(μ) of the hollow fibers exclusive of the fin portions in the hollowfiber bundle, and N stands for the number of the hollow fibers containedin the shell.

The fluid separator of the present invention is preferably used as ablood dialyzer. In this case, in view of the blood treatment efficiency,it is preferred that the water permeation capacity UFR of the hollowfiber exclusive of the fin portion be in the range of from 1.0 to 30.0ml/(m². hr.mmHg), especially 3.0 to 10.0 ml/(m².hr.mmHg).

The value y is generally called "hollow fiber filling ratio", and it ispreferred that the value y be 40 to 60%. If finned hollow fibers areused, the filling ratio for attaining the predetermined blood treatmentefficiency, that is, the number of the hollow fibers to be filled, canbe reduced. As the number of the fins is large, the degree of thereduction is increased, so far as the value x is within theabove-mentioned range. The effect is high as the number of the hollowfibers is great, though the behavior is influenced to some extent by thefin height.

This reduction of the hollow fibers in the blood treating shell is duemainly to the fact that the dialysis performance is highly improved byincrease of the coefficient of the transport of the solute to thedialyzing solution at the time of the dialysis.

As is well-known, the coefficients of the transport of urea, creatineand uric acid in blood to the dialyzing solution participate in thedialysis performance. The overall solute transport coefficient Ko of thedialyzer has the following relation to the solute transport coefficientKb in the boundary layer on the blood side, the solute transportcoefficient Km in the hollow fiber membrane and the solute transportcoefficient Kd in the boundary layer on the dialyzing solution side:

    1Ko=1/kb+1/Km+1/Kd

In order to increase Ko, it is necessary that means should be adoptedfor increasing Kb, Km and Kd.

In the blood treatment satisfying the requirement of the above formula(I) according to the present invention, when the hemodialysis is carriedout at 37° C. in the state where the average flow rate of the dialyzingsolution in the hollow fiber bundle-filled shell is 1.8 cm/sec, if thesolute transport coefficient Kd (cm/min) of urea on the dialyzingsolution side is at least 1/12, preferably at least 1/5, especiallypreferably at least 1/2 the flow of the dialyzing solution is veryuniformly distributed in the hollow fibers. This effect sufficientlycovers the reduction of the effective area in the root portions of thefins and the dialysis performance of the dialyzer as a whole is muchimproved over the dialysis performance attained when no fins are formedon the hollow fibers.

Since fins are formed on the peripheral portions of hollow fibers,contact of the hollow fibers with one another or close approaching ofthe hollow fibers to one another is prevented and the dialyzing solutionis uniformly distributed in the hollow fibers. Furthermore, the increaseof the blood speed by the reduction of the number of the hollow fibersmakes a contribution to the increase of Kb. Namely, Kb can be increasedby increasing the blood speed in the hollow fibers, and if the number ofhollow fibers is decreased in a long shell, the treatment isadvantageously accomplished.

In the case where the fluid separator of the present invention is ablood dialyzer in which blood is passed through the interiors of thehollow fibers, at an ordinary blood speed of 200 cc/min, the number ofhollow fibers is smaller than in an ordinary blood dialyzer and 3000 to11000, preferably 4000 to 9000.

It is preferred that the thickness of the hollow fiber used in thepresent invention, except the fin portion, be small, that is, the valueof Km be large. Incidentally, this hollow fiber shows a goodform-retaining property at the time of the preperation or when thehollow fiber is assembled in the fluid separator or the fluid separatoris actually used.

Incidentally, even in the case where finned hollow fibers are used,there is a lower limit of the hollow fiber filling ratio y representedby the above-mentioned formula (I), and if the number of the hollowfibers is too small, there is present a hollow fiber-absent space and ashort circuit is formed for the dialyzing solution. It has been foundthat the critical limit is expressed by the left-hand side of theformula (I). On the other hand, if the ratio y is too high, insertion orfilling of hollow fibers in the shell becomes difficult, and it has beenfound that when hollow fibers are excessively filled in the shell, alocal short circuit of the dialyzing solution is formed and thedialyzing efficiency is rather reduced. This upper limit is expressed bythe right-hand side of the formula (I). It has been found that when therequirement of the formula (I) is satisfied, the finned hollow fibersshow excellent effects over ordinary circular hollow fibers.

When the fluid separator of the present invention satisfying the aboverequirement is compared with a fluid separator comprising ordinarycircular hollow fibers, the dialysis efficiency is greatly improved. Forexample, in case of the fluid separator of the present invention, whenthe average blood speed in the hollow fibers is 1.2 cm/sec, the averageflow rate of the dialyzing solution outside the hollow fibers is 1.8cm/sec and the temperature is 37° C., the solute transport coefficientKo of urea is at least 1/15 cm/min, preferably at least 1/6 cm/min.

As the material of the hollow fiber membrane used for the fluidseparator of the present invention, there can be mentioned celluloses,cellulose esters, polyamides, polyacrylonitrile, polycarbonates,polymethyl methacrylate, polyolefins, polysulfones polyethersulfones andcopolymers and mixtures thereof.

As the resin used as the partition wall, any of resins customarily usedfor fluid separators of this type may be used. For example, there can bepreferably used a polyurethane resin and a silicone resin.

In the fluid separator of the present invention, it is preferred thatthe distribution of hollow fibers in at least one of sections verticalto the axial direction of the hollow fiber bundle in the partition wallof the shell to which both the ends of the hollow fiber bundle aresecured by a resin should satisfy the requirement represented by thefollowing formula (III):

    σ/X≦0/1                                       (III)

wherein X stands for the average value of the number of hollow fiberspresent in an area of 4 mm² at optional twenty points where hollowfibers are present in said at least one section, and o indicates thestandard deviation of said number of the hollow fibers.

At least one section vertical to the axial direction of the hollow fiberbundle in the partition wall may be any hollow fiber-present section inthe partition wall to which both the end portions of the hollow fiberbundle are secured by a resin. In the blood-purifying fluid separator ofthe type where blood is passed through the interiors of hollow fibers,it is preferred that said at least one section be a section in thepartition wall on the blood introduction side, especially a section ofthe blood introduction portion to which the hollow fibers are opened. Incase of a dialysis type blood purifying device provided with an inletfor a dialyzing solution, it is preferred that said at least one sectionbe one in the partition wall in the vicinity of the inlet for adialyzing solution.

In the formula (III), X represents the average value of the number x ofhollow fibers present in an area of 4 mm ² at optional twenty pointswhere hollow fibers are present in said at least one section, and oindicates the standard deviation of the number x of the hollow fibers.The measurement of the number x of the hollow fibers is made on thehollow fiber-present portion of the predetermined section of thepartition wall, which is exposed by cutting according to need, by usinga scaled optical microscope.

When the section of one hollow fiber is divided by a scale of an area of4 mm² in the microscope, the proportion of the portion included in thescale to the entire section of the hollow fiber is calculated, and thesum of the hollow fibers included in the scale is counted. Accordingly,the value of x is not an integer but is ordinarily a number having adecimal fraction. In the cut section of the hollow fibers and adhesiveand there is established the relation of σ/X≦0.1 between the averagevalue X and the standard deviation σ as pointed out hereinbefore, thedialyzing efficiency of the fluid separator is increased and blood ishardly left in the separator. If the above requirement is not satisfied,vacant portions where hollow fibers are hardly present and portionswhere hollow fibers densely gather appear here and there, and channelingis readily caused in the blood and dialyzing solution. When therequirement of σX≦0.05 is satisfied, the amount of the residual blood isespecially-reduced and excellent effects can be attained.

When the fluid separator of the present invention is used as a blooddialyzer, it is preferred that most of hollow fiber membranes bemembranes of hollow fibers having at least two fins extending in thelongitudinal direction on the periphery thereof and the total effectivemembrane area S (m²), the ultrafiltration coefficient UFR(ml/m².hr.mmHg) and the urea clearance (or urea dialysance) CLU (ml/min)of the dialyzer should satisfy any of the following requirements (IV)through (VI):

(IV) UFR/S ≦6 and CLU/UFR ≧30 in case of 0.5 ≦S ≧1.0,

(V) UFR/S ≦5 and CLU/UFR ≧30 in case of 1.0 <S ≦1.6, and

(VI) UFR/S ≦4 and CLU/UFR ≧25 in case of 1.6 <S <2.5.

In accordance with one preferred embodiment of the present invention,there is provided a fluid separator as set forth above, wherein thehollow fiber membranes are composed of a cellulose, the thickness of themembranes exclusive of the fin portions is less than 30 μ and theeffective membrane area is at least 0.5 m². In accordance with anotherpreferred embodiment of present invention, there is provided a fluidseparator as set forth above, wherein the hollow fiber membranes arecomposed of a semi-synthetic or synthetic polymer such as a celluloseester, polymethyl methacrylate, an ethylene/vinyl alcohol copolymer,polyacrylonitrile, a polycarbonate polysulfone or a polyethersulfone,the thickness of the hollow fiber membranes exclusive of the finportions is less than 50 μ and the effective membrane area is at least0.7 m².

If the value of UFR/S exceeds the upper limit in each case, especiallyif the value of UFR/S exceeds 6, control of removal of water isdifficult at the dialysis and excessive removal of water is oftencaused. If the value of CLU/UFR is smaller than the lower limit in eachof the cases (IV) to (VI), especially if the value of CLU/UFR is smallerthan 25, no satisfactory dialysis effect can be attained, and wastes aregradually accumulated in the body and so-called underdialysis, is oftencaused to occur.

Selective-permeable hollow fibers used for the fluid separator of thepresent invention may have a plurality of crimps according to need. Itis preferred that the amplitude of the crimps be 1 to 500%, especially20 to 200%, of the outer diameter d of the hollow fibers. If theamplitude is smaller than 1% of d, no particular effect is attained bycrimping. If the amplitude exceeds 500% of d, the flow resistance in thehollow fibers is increased and when the hollow fibers are used for theblood treatment, blood is readily left in the hollow fibers. It ispreferred that the wavelength of crimps in the hollow fibers of thepresent invention be 5 to 1000 times, especially 50 to 500 times, theouter diameter d of the hollow fibers.

If desired, in the fluid separator of the present invention,selective-permeable hollow fibers may have a plurality of fins in whichthe average height is substantially different in the respective finsand/or the distance between two adjacent fins is substantiallydifferent. Furthermore, selective-permeable hollow fibers have at leastthree fins, a plurality of selective-permeable hollow fibers are bondedtogether through the fins, and the average height is substantiallydifferent in the fins other than the fins bonding the hollow fibersand/or the distance between two adjacent fins is substantiallydifferent.

A hollow fiber having a plurality of fins in which the average height ofthe highest fin is 1.5 to 10 times, especially 2 to 5 times, the averageheight of the lowest fin is especially preferred because contact orclose approaching is not caused in the hollow fibers, a sealed space ishardly formed between the fins or between the fin and the hollow fiberwall and the fluid can flow freely.

Furthermore, it is preferred that in a plurality of fins, the maximumvalue of the distance between two adjacent fins be 1.5 to 10 times,especially 2 to 5 times, the minimum value of the distance between twoadjacent fins. In case of hollow fibers having such fins, in a fluidseparator having these hollow fibers filled therein, contact or closeapproaching is hardly caused and a sealed space is hardly formed betweenthe fins or between the fin and the hollow fiber membrane, and theflowability of the fluid outside the hollow fiber is highly improved.The distance between two adjacent fins is a shortest distance betweenthe centers of two adjacent fins along the periphery of the outer wallof the hollow fiber.

Selective-permeable finned hollow fibers valuably used for the fluidseparator of the present invention can be advantageously prepared byextruding a spinning liquid from a spinning nozzle for finned hollowfibers, which has an appropriate configuration, so that the viscosity ofthe liquid at the nozzle is 100 to 10,000 poise (hereinafter, referredto as P).

The spinning solution for hollow fibers is in the liquid state, andafter it is extruded from the spinning nozzle, the liquid of the finportion tends to gather, toward the root because of the surface tension.In order to control this tendency, it is necessary that the viscosity ofthe spinning liquid at the time of extrusion from the nozzle should beincreased and the time required for formation of the shape of the hollowfiber should be shortened. As the result of the research made with aview to solving this problem, it was found that in order to obtainnon-circular finned hollow fibers which can be used for purification ofblood and have such a shape that when used for the hemodialysis, theflow of the dialyzing solution is uniformalized and a sufficientdialyzing effect is attained without substantial reduction of theeffective area of the hollow fiber membrane, it is important that themelt viscosity of the spinning liquid at the spinning nozzle should be100 to 10,000 P.

A spinneret shown in FIG. 6 is ordinarily used for spinning of finnedhollow fibers. A gas or liquid for formation of the hollow portion isextruded from a portion 1, and the spinning liquid of the hollow fibermembrane forming material is extruded from a portion 2. Then, theextruded spinning liquid is cooled and solidified and the shape of thehollow fiber is fixed. Nozzles as shown in FIGS. 7 and 8 may be used asmeans for narrowing the root of the fin portion. If the viscosity of thespinning liquid is lower than 100 P, the shape of the fin is notnormally fixed and the intended object is not sufficiently attained.

On the other hand, if the melt viscosity of the spinning liquid is toohigh and exceeds 10,000 P, the pressure loss at the extrusion hole ofthe spinneret is increased, and a mechanical problem arises and it isimpossible to impart smooth drafting (drawing) to the extrudate.Accordingly, breaking is readily caused during spinning and thethickness unevenness is caused, and stable spinning is impossible.

It is especially preferred that the viscosity of the spinning liquid atthe spinning nozzle be 300 to 2,000 P, particularly 500 to 1,500 P. Ifthe viscosity of the spinning liquid is within this range, the shape ofthe fins is correctly fixed and spinning can be performed stably. In thepresent invention, by the viscosity of the spinning liquid at thespinning is meant the viscosity of the spinning liquid at the spinningnozzle, strictly at the outlet of the spinning nozzle.

If the time required for solidification of the extruded hollow fiber istoo long even though the viscosity of the spinning liquid is within anappropriate range at the extrusion from the nozzle, deformation ispromoted by the surface tension of the spinning liquid and a sharp shapecannot be given to the fin. In order to sharply fix the shape of theextrudate, it is preferred that the extrudate be promptly cooled andsolidified after the extrusion. In the ordinary melt spinning, theso-called solidification point where the extrudate loses the flowabilityand attenuation by the winding tension is not advanced any more is apartby about 10 cm from the nozzle. However, it was found that in order toobtain a finned hollow fiber for purification of blood, as intended inthe present invention, it is preferred that the solidification point belocated within 40 cm from the top end of the nozzle. It is especiallypreferred that the distance between the solidification point and the topend of the nozzle be 5 to 40 cm, particularly 10 to 25 cm. If thedistance is within this range, spinning can be performed very smoothly.

The draft ratio as another spinning condition is preferably 20 to 200and especially preferably 40 to 100 in the present invention. If thedraft ratio is within this range, a hollow fiber provided with finshaving a predetermined shape can be obtained stably.

The spinning liquid used in the preparation process of the presentinvention is composed mainly of a thermoplastic polymer, but additivesmay be incorporated in the spinning liquid. As the thermoplasticpolymer, there are preferably used a cellulose ester, polyethylene,polypropylene, polyethylene terephthalate, polymethyl methacrylate, anda polyether sulfone. As the additive, there can be mentioned polyolssuch as polyalkylene glycols, glycerol and glycols having ethyleneand/or propylene chain in the molecule, plasticizers such as sulforane,caprolactone, dimethylsulfoxide and mixtures thereof, and thermoplasticpolymers as mentioned above and other thermoplastic polymers. Thecomposition of the spinning liquid is appropriately selected accordingto the required selective permeability and the viscosity at the spinningstep.

In the process of the present invention, the selective permeability isimparted by drawing of the spun and solidified hollow fiber, extractionremoval of the additives or the chemical treatment such assaponification. An appropriate treatment is selected according to thecomposition of the spinning liquid, but any of customary treatments maybe adoped and the treatment method is not particularly critical.

The shape of size of the finned hollow fiber of the present invention isnot particularly critical. The number of fins is 1 to 10 and preferably2 to 7, and the outer diameter d is 100 to 500 μ and preferably 200 to300 μ. The thickness (h) of the fin-free portion is 5 to 50 μ andpreferably 10 to 40 μ. The height H of the fin is practically in therange of from 10 to 65 μ.

In accordance with the present invention, there is provided a cellulosetype hollow fiber that is very suitably used for the above-mentionedfluid separator. This fiber is a cellulose type hollow fiber having onthe periphery thereof fins extended in the longitudinal direction andhaving a selective permeability, wherein the average degree ofpolymerization of a cellulose type polymer as the main constituent is atleast 150 and the ratio H/W of the fin height H to the fin width W is atleast 0.5.

The selective-permeable cellulose type hollow fiber of the presentinvention comprises a cellulose type polymer as the main constituent. Asthe cellulose type polymer, there can be mentioned cellulose, celluloseacetates such as cellulose diacetate and cellulose triacetate, cellulosepropionate, cellulose butyrate, cellulose acetate butyrate, andcellulose nitrate. These cellulose type polymers may be used singly orin the form of a mixture of two or more of them. Cellulose and celluloseesters such as cellulose diacetate and cellulose triacetate arepreferred as the cellulose type polymer.

The cellulose type polymer used in the present invention ischaracterized in that the average degree of polymerization is at least150. By the average degree of polymerization referred to herein is meanta value obtained by dividing the weight mean molecular weight by themolecular weight of the recurring unit of the polymer. As means fordetermination of the weight mean molecular weight, there can bementioned the gel filtration chromatography, the measurement of thecoefficient of viscosity, the measurement of the sedimentationequilibrium and the light scattering method.

If the average degree of polymerization of the cellulose type polymer islower than 150, a selective-permeable cellulose type hollow fiber havingfins having a large height can hardly be obtained, and a hollow fiberhaving a low re-wet elongation can hardly be obtained. It is preferredthat the average degree of polymerization be at least 160, especially atleast 170. The upper limit of the average degree of polymerization isnot particularly critical, but from the practical viewpoint, it ispreferred that the degree of polymerization be lower than 230,especially lower than 200.

The cellulose type hollow fiber of the present invention comprises asthe main constituent a cellulose type polymer as described above. Thecontent of the cellulose type polymer is at least 50%, preferably atleast 70%. Other component is not particularly critical, and forexample, there can be mentioned organic polymers such as polymethylmethacrylate, polyethylene, polypropylene, polyacrylonitrile, andpolyvinyl alcohol.

The cellulose type hollow fiber of the present invention has on theperiphery thereof fins extended in the longitudinal direction, and thehollow fiber is characterized in that the ratio H/W of the fin height Hto the fin width W is at least 0.4 ,preferably at least 0.5. If theratio H/W is lower than 0.5, contact of hollow fibers with one anothercan hardly be prevented when they are filled in the fluid separator. Itis preferred that the ratio H/W be at least 0.8. Incidentally, the finwidth W is the width of the fin at a position of 1/2 of the fin heightH.

The number x of fins in the hollow fiber of the present invention is atleast 1. However, if the number x is 11 or larger, reduction of theeffective membrane area at the root portions of the fins becomesprominent, and the solute dialyzing capacity and water permeability ofthe blood dialyzer are drastically reduced. Accordingly, the number x isgenerally in the range of from 1 to 10, and it is preferred that thenumber x be 2 to 8, especially 3 to 7.

It is preferred that the outer diameter d of the hollow fiber of thepresent invention be 100 to 400 μ, especially 200 to 300 μ, and that thethickness h of the fin-free portion of the hollow fiber be 5 to 50 μ,especially 5 to 30 μ, particularly especially 10 to 25 μ. Furthermore,it is preferred that the fin height H be 5 to 100 μ, especially 9 to 60μ, and that the ratio H/d be 0.5 to 3, especially 1 to 2. Moreover, itis preferred that the ratio H/d of the average height H (μ) of fins tothe average outer diameter d of the hollow fiber exclusive of the finportion be 0.01 to 1, especially 0.02 to 0.5, particularly especially0.03 to 0.2.

It is preferred that the width of the root portion of the fin benarrower than that of the upper portion. However, ordinarily, the rootportion is readily expanded by the surface tension of the spinningliquid after extrusion of the spinning liquid from the nozzle. The widthof the root portion is 15 to 50 μ and preferably 20 to 40 μ.

If the width of the root portion is within this range, the circularityof the hollow fiber is good and coagulation of blood is hardly caused inthe hollow fiber or blood is hardly left in the hollow fiber. If onehollow fiber has a plurality of fins, the respective fins may be thesame or different in the height H or width W. Furthermore, at leastparts of the fins may be spirally extended on the outer surface of thehollow fibers, or the fins may be partially discontinuous.

The hollow fiber of the present invention is characterized in that there-wet elongation is lower than 3%. If the re-wet elongation exceeds 3%,when the hollow fiber is filled in the fluid separator in thesubstantially dry state and is then re-wetted with water or an aqueoussolution, the hollow fiber is excessively elongated, with the resultthat local contact or close approaching is caused among hollow fibersand the hollow fibers are readily bent or curved. It is preferred thatthe re-wet elongation be lower than 2.5%. The lower limit of the re-wetelongation is not particularly critical, but if the re-wet elongation ishigher than 0.5%, especially higher than 1%, the hollow fiber shows anappropriate elongation when re-wetted in the fluid separator and thehollow fiber bundle is expanded throughout the fluid separator.Incidentally, the re-wet elongation α is determined by measuring thelength l of the hollow fiber in the substantially dry state under a loadof about 0.1 g/de, measuring the length l₁ of the hollow fiber in thewet state under a load of about 0.01 g/de and making the calculationaccording to the formula of ##EQU1## The substantially dry state means astate of a glycerol content of 30 to 60% and a water content of 0.5 to30%, preferably 0.5 to 9%, which is produced by immersing the hollowfiber in an aqueous solution of glycerol and then drying the hollowfiber.

It is preferred that the wet tensile strength of the hollow fiber of thepresent invention be at least 0.3 g/de, the shape-retaining property ispoor, and the hollow fiber is readily broken in the preparation processor at the step of assembling the fluid separator. This phenomenon isespecially prominent when the outer diameter d is 100 to 300 μ and thethickness is 5 to 25 μ. The upper limit of the wet tensile strength isno particularly critical. For example, the wet tensile strength isordinarily up to 0.8 g/de.

In the hollow fiber of the present invention, it is preferred that theflexural rigidity in the substantially wet state be at least 200 mg/200fibers, and that the rupture pressure as the factor indicating thepressure resistance when the interior of the hollow fiber in the wetstate is compressed be at least 4 kg/cm². If the flexural rigidity andrupture pressure are within the above-mentioned ranges, handling ofhollow fibers is facilitated at the step of assembling the fluidseparator, the lengths of the hollow fibers are readily uniformalized inthe fluid separator and the hollow fibers are hardly broken while thefluid separator is actually operated under compression.

The cellulose type hollow fiber of the present invention has a selectivepermeability to fluids. For example, when the cellulose type hollowfiber is used for the hemodialysis, it is preferred that the waterpermeation capacity UFR of the hollow fiber exclusive of the fin portionbe 1.0 to 30.0 ml/(m².hr.mmHg), especially 3.0 to 10.0 ml/(m².hr.mmHg).

The cellulose type hollow fiber of the present invention having such aselective permeability exerts an excellent capacity in the fluidseparation. The fluid separation is not particularly critical. Forexample, there can be mentioned the liquid separation such as dialysis,ultrafiltration and reverse osmosis, and the gas separation for anartificial lung. The hollow fiber of the present invention is especiallysuitable for the separation utilizing the difference of the diffusionspeed, such as the dialysis in which a good flow state of the fluidoutside the hollow fiber is required, or the blood treatment in whichsmooth flowing within the hollow fiber is necessary. In the hollow fiberof the present invention, other membrane having a fluid separatingcapacity may be formed as a supporting membrane on the inner or outermembrane surface.

The process for the preparation of the cellulose type hollow fiberaccording to the present invention is characterized in that a liquidmixture comprising a cellulose ester polymer having an average degree ofpolymerization of at least 150 and other additives is used as thespinning liquid, and the spinning liquid is extruded together with acore agent from a spinneret for a finned hollow fiber in the state wherethe viscosity is in the range of from 100 to 10,000 P. It is preferredthat the viscosity be 200 to 5,000 P, especially 500 to 3,000 P, in caseof melt spinning, and the viscosity be 200 to 5,000 P in case ofsemi-dry semi-wet spinning or wet spinning. The melt viscosity ismeasured by using a flow tester, and the liquid viscosity is measured bya rotational viscometer.

Any of melt spinning, wet spinning and semi-dry semi-wet spinning can beadopted as the preparation process. In case of the melt spinningprocess, the spinning liquid is a heated melt, and the melt is extrudedfrom the spinneret and cooled to form a solidified finned hollow fiber.Then, at least a part of the additive in the finned hollow fiber isextracted and removed by a solvent incapable of dissolving the celluloseester polymer but capable of dissolving the additive, and if necessary,the hollow fiber is subjected to an alkali treatment. Thus, a finnedcellulose type hollow fiber having a selective permeability can beobtained. In this melt spinning process, if the time required forsubstantial solidification of the hollow fiber by cooling from the pointof extrusion of the heated solution, from the spinneret is less than 10second and more preferably less than 5 seconds, spinning can beperformed stably while retaining a good shape of the fin. By the term"solidification" used herein is meant the state where the spun fiberloses the flowability and attenuation is not advanced any more by thewinding tension. The solidification is determined by taking out a fiberbeing spun by a clipper and measuring the point of termination of theattenuation.

A specific example of the melt spinning process will now be described. Aheated spinning liquid (melt or solution) is extruded in a gas orspinning bath according to customary procedures, for example, from ahollow hole of a spinneret having a notch in the periphery of a doubleannular portion, and spinning is conducted while retaining the hollowportion by filling a gas or liquid substantially incapable of dissolvingthe membrane material or reacting therewith into the central hollowportion. For example, a plasticizer such as polyethylene glycol is addedas the additive to flakes of cellulose diacetate, and the mixture ismolten and the melt is extruded into air from a hollow hole of thespinning nozzle. Then, nitrogen gas is blown into the central portionand simultaneously, the extrudate is cooled. The plasticizer is removedfrom the obtained hollow fiber extrudate If necessary, a saponificationtreatment is carried out with caustic soda. Thus, a finned hollow fiberhaving a solute selective permeability is prepared.

As the plasticizer, there can be mentioned polyalkylene glycols such aspolyethylene glycol, glycols having an ethylene/propylene chain in themolecule, glycerol, diglycerol, sulforanes, caprolactones, anddimethylsulfoxide. The amount of the plasticizer added to the spinningsolution is appropriately determined according to the intended use ofthe hollow fiber. For example, when the hollow fiber is used for thehemodialysis, it is preferred that the amount of the plasticizer be 20to 70% by weight, especially 25 to 60% by weight.

When the wet spinning or semi-dry semi-wet spinning process if adoptedfor the preparation of the hollow fiber of the present invention, theadditive comprises a solvent for the cellulose ester polymer, and thespinning solution is extruded from the spinneret and immersed in acoagulating bath. In order to form pores in the hollow fiber membranewith ease, it is preferred that a poor solvent for the cellulose typepolymer or a metal salt be added as the pore-forming agent to theabove-mentioned additive. In this spinning process, it is preferred thatthe time of from the point of the extrusion from the spinneret to thepoint of immersion in the coagulating bath be up to 5 seconds,especially up to 1 second. The kinds and amount of the solvent, the poorsolvent and the metal salt and the compositions of the coagulatingsolution and the core agent may be appropriately selected according tothe intended use of the hollow fiber membrane. A cheap solvent capableof dissolving the cellulose type solvent may be used as the solvent, anda solvent hardly capable of dissolving the cellulose type polymer may beused as the poor solvent

In the process for the preparation of the hollow fiber of the presentinvention, it is sometimes preferred that the spinning be carried out ata winding speed higher than the extrusion speed. In this case, it ispreferred that the draft ratio be 30 to 200 in case of the melt spinningprocess and 5 to 50 in case of the wet or semi-dry semi-wet spinningprocess. In the case of the melt spinning, the draft ratio refers tothat before the solidification of the spun filament. Optionally, thespun filament may further be drafted after the solidification thereof,but usually, such further drafting is not necessary.

The fluid separator of the present invention can be used for the liquidseparation such as dialysis, ultrafiltration, precision filtration andreverse osmosis, and the gas separation such as oxygen enriching andartificial lung. The fluid separator of the present invention issuitable as a body fluid treatment device such as an artificial kidney,an artificial liver, a plasma separating device, an abdominal dropsytapping device or an artificial lung and is especially suitable as ablood dialyzer.

The present invention will now be described in detail with reference tothe following examples that by no means limit the scope of theinvention.

EXAMPLES 1 THROUGH 8 AND COMPARATIVE EXAMPLES 1 THROUGH 4

To 100 parts of cellulose diacetate was added 50 parts of polyethyleneglycol (having a molecular weight of 200), and the mixture was molten at230° C. and the melt was spun from a spinneret for a finned hollow fiberand also from an ordinary annular fin-free spinneret. After cooling, theextrudates were immersed in a 2% aqueous sodium hydroxide solution at70° C. to saponify them. Then, they were washed with water, immersed inan approximately 50% aqueous glycerol solution, and dried with hot airto obtain circular finned hollow fibers and circular fin-free hollowfibers shown in Table 1 In each of these fibers, in a wet state, theinner diameter was about 200 μ and the thickness of the fin-free portionwas 22 to 25 μ.

A predetermined number, shown in Table 1, of the so-obtained hollowfibers were filled into a circular tube shell at an occupancy ratio y toconstruct a blood dialyzer, and the in vitro ultrafiltration performanceand dialysis performance were measured. Furthermore, the easiness ordifficulty of the operation of filling the fibers into the shell waschecked.

The dialysis performance (dialysance) 1/Kd was measured at 37° C undersuch conditions that the average flow rate of blood in the hollow fiberswas 1.2 cm/sec and the average flow rate of the dialyzing solution was1.8 cm/sec. The obtained results are shown in Table 1. From theseresults, it is seen that the hollow fiber of the present inventionprovides a blood purifier having highly improved ultrafiltrationperformance and dialysis performance and has a good adaptability toinsertion in the shell.

Incidentally, 1/Kd was determined in the following manner. Namely, 1/Kowas calculated according to the following equation: ##EQU2## wherein Qbstands for the flow rate (cm^(3/) min) of blood side, Qd stands for theflow rate (cm^(3/) min) of the dialyzing solution side, Da stands forthe dialysance measured according to the dialyzer performance evaluationstandard stipulated at the meeting of the Japanese Artificial OrganAssociation held in Sep. 1982, and A stands for the effective membranearea (cm²) of the hollow fiber, that is, the membrane area based on theinner diameter in the wet state, exclusive of the root portion of thefin, not participating in the transport of the solute, in case of thefinned hollow fiber.

The measurement of 1/Km +1/Kb was carried out in a manner of a modelunder such conditions that 1/Kd was substantially zero. The value 1/Kdof the blood purifier was calculated from the equation of Kd =1/Ko -(1/Km +1/Kb).

EXAMPLES 9 THROUGH 11

In the same manner as described in Example 1, 132 parts of polyethyleneglycol (having a molecular weight of 200) was added to 100 parts ofcellulose diacetate, the spinning liquid was spun into circular hollowyarns having three fins and the polyethylene glycol was dissolved outand removed by the hot water treatment. Then, the filaments were washedwith hot water, immersed in an aqueous glycerol solution, and dried withhot air to obtain hollow fibers for the dialysis. In the so-obtainedhollow fibers, the inner diameter was about 200 μ and the membranethickness of the fin-free portion was about 25 μ. Dialyzers wereassembled by using these hollow fibers as indicated in Table 1, and themeasurement was conducted in the same manner as described in Example 1.The obtained results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                       Number    Effective     Urea                               Example                                                                              Number      of Hollow Membrane l/K.sub.D                                                                          Dialysance                                                                          Adaptability                 No.    of Fins                                                                            α                                                                          W (μ)                                                                          Fibers                                                                              y (%)                                                                             Area (m.sup.2)                                                                      UFR                                                                              (cm/min)                                                                           (ml/min)                                                                            to Insertion                 __________________________________________________________________________    Example 1                                                                            1    0.14                                                                             28  10,300                                                                              53  1.3   5.2                                                                              8    160   good                         Example 2                                                                            3    0.14                                                                             28  9,000 48  1.1   4.4                                                                              5    167   good                         Example 3                                                                            6    0.14                                                                             28  8,500 44   0.95 3.8                                                                              2    168   good                         Example 4                                                                            10   0.14                                                                             28  7,800 40   0.82 3.3                                                                              1    165   good                         Example 5                                                                            3    0.06                                                                             25  9,000 48  1.1   4.4                                                                              8    163   good                         Example 6                                                                            3    0.18                                                                             30  9,000 48  1.1   4.4                                                                              4    165   good                         Example 7                                                                            6    0.14                                                                             28  7,000 36   0.74 3.0                                                                              10   160   good                         Example 8                                                                            3    0.14                                                                             28  11,000                                                                              58  1.3   5.3                                                                              3    169   good                         Example 9                                                                            3    0.12                                                                             20  9,700 46   0.87 5.1                                                                              7    168   good                         Example 10                                                                           3    0.12                                                                             30  9,000 48  1.1   5.5                                                                              6    159   good                         Example 11                                                                           3    0.12                                                                             30  9,000 48  1.1   6.0                                                                              7    160   good                         Comparative                                                                          0    -- --  9,000 48  1.2   4.8                                                                              17   145   good                         Example 1                                                                     Comparative                                                                          3    0.14                                                                             28  6,800 35  0.8   3.2                                                                              21   142   good                         Example 2                                                                     Comparative                                                                          3    0.14                                                                             28  12,000                                                                              62  1.4   5.6                                                                              18   148   bad                          Example 3                                                                     Comparative                                                                          12   0.14                                                                             28  7,800 40  0.6   2.4                                                                              10   152   good                         Example 4                                                                     __________________________________________________________________________

EXAMPLE 12 THROUGH 14

The procedure as in Example 9 was repeated, but using a differentspinneret, to obtain circular hollow fibers having six fins and amembrane thickness of the fin-free portion of about 15 μ. Dialyzers wereassembled by using the hollow fibers as indicated in Table 1, and themeasurement was conducted to obtain the results as shown in Table 1.

                                      TABLE 2                                     __________________________________________________________________________    Example                                                                             Number      Number of   Effective Mem-                                                                             l/K.sub.D                                                                          Urea Dialy-                                                                           Adaptability          No.   of Fins                                                                            α                                                                          W (μ)                                                                          Hollow Fibers                                                                         y (%)                                                                             brane Area (m.sup.2)                                                                   UFR (cm/min)                                                                           sance (ml/min)                                                                        to                    __________________________________________________________________________                                                            Insertion             Example 12                                                                          6    0.06                                                                             25  12,000  45  1.5      8.5 4    170     good                  Example 13                                                                          6    0.10                                                                             30  12,000  45  1.5      8.0 5    173     good                  Example 14                                                                          6    0.15                                                                             35  12,000  45  1.5      7.4 5    175     good                  __________________________________________________________________________

EXAMPLES 15 THROUGH 19 AND COMPARATIVE EXAMPLES 5 THROUGH 7

To 100 parts of cellulose diacetate was added 50 parts of polyethyleneglycol (having a molecular weight of 300), and the mixture was molten at230° C. and spun from spinnerets for hollow fibers having 3 or 6 finsand an ordinary fin-free annular spinneret. The post treatments werecarried out in the same manner as described in Example 1 Thus, hollowfibers having 3 or 6 fins and fin-free hollow fibers were prepared. Ineach of these hollow fibers, the inner diameter was 200 μ and themembrane thickness was 25 μ. In the finned hollow fibers, the fin heightwas 35 μ. These hollow fibers were saponified to obtain hollow fibermembranes.

Three kinds of the so-obtained hollow fibers were filled in shellshaving the same size at filling ratios shown in Table 3 to assembledialyzers. The in vitro dialysis performance was determined.Furthermore, bovine blood was diffused in the dialyzers and afterremoval of blood, the amounts of blood left in the dialyzers weremeasured. Moreover, with respect to 50 assembled dialyzers, the numberof the dialyzers where leaking (tube plate leak) was caused by minutespaces formed in the non-bonded portions among the hollow fibers in thepartition wall was checked. The obtained results are shown in Table 3.

In checking the leaking, water was filled on the dialyzing solutionside, an air pressure of 0.5 K was applied to the blood side, andformation of bubbles was examined.

Incidentally, the urea clearance was determined according to thedialyzer performance evaluation standard specified at the meeting of theJapanese Artificial Organ Association held in September 1982. The ureaclearance was 160 to 167 in Examples 15 through 19, while the ureaclearance was 155 to 158 in Comparative Examples 5 through 7.

From the results shown in Table 3, it was found that the dialysisperformance is improved according to the present invention and theamount of blood left after diffusion can be reduced according to thepresent invention. Moreover, it was confirmed that in the dialyzer ofthe present invention, occurrence of leaking by minute spaces in theadhesive-bonded tube plate of the partition wall is prevented.

                  TABLE 3                                                         ______________________________________                                                      Occu-           Amount                                                        pancy           (ml) of                                                                              Tube Plate                                      Number Ratio           Residual                                                                             Leak Ratio                                      of Fins                                                                              (%)     σ/- x                                                                           Blood  (%)                                      ______________________________________                                        Example 15                                                                             1        53      0.09  0.4    1.9                                    Example 16                                                                             3        50      0.07  0.3    0                                      Example 17                                                                             3        46      0.08  0.3    0.5                                    Example 18                                                                             6        47      0.05  0.1    0                                      Example 19                                                                             6        40      0.06  0.2    0                                      Comparative                                                                            0        53      0.15  1.1    8.0                                    Example 5                                                                     Comparative                                                                            1        53      0.12  0.8    5.1                                    Example 6                                                                     Comparative                                                                            3        46      0.11  0.9    4.5                                    Example 7                                                                     ______________________________________                                    

EXAMPLES 20 THROUGH 28 AND COMPARATIVE EXAMPLES 8 THROUGH 10

To 100 parts of cellulose diacetate was added 45 parts of polyethyleneglycol (having a molecular weight of 400), and the mixture was molten at220° C. and spun through spinnerets for circular hollow fibers or finnedhollow fibers. The spun hollow fibers were saponified by sodiumhydroxide to convert cellulose diacetate to cellulose, and thesaponified fibers were wound. The membrane thickness (in the wet state)of the fin-free portion and the number of fins were as shown in Table 4.These factors were adjusted based on the kind of the spinneret and theextrusion rate. The so-obtained cellulose hollow fibers were filled inshells differing in the size, and the in vitro UFR and urea clearanceCLU . were determined. The obtained results are shown in Table 4. Thevalues of UFR and clearance CLU were obtained according to the standardof the Japanese Artificial Organ Association. From the results shown inTable 4, it was found that in the dialyzer according to the presentinvention, the clearance is high and UFR is at an appropriate level,even if the membrane area is small. Namely, in the dialyzer according tothe present invention, a good balance is maintained between theclearance and UFR and the dialysis performance is high.

EXAMPLE 29 AND COMPARATIVE EXAMPLE 11

Hollow fibers shown in Table 4 were prepared in the same manner asdescribed in Example 20, and blood dialyzers having an effective area of0.8 m² were assembled by using these hollow fibers. With respect to eachdialyzer, the in vitro performance was determined. The obtained resultsare shown in Table 4. From the results shown in Table 4, it was foundthat in case of fin-free hollow fibers having a membrane thickness of 35μ, as prepared in Comparative Example 11, the dialysis performance islow and the dialyzer has no practical utility.

EXAMPLE 30 AND COMPARATIVE EXAMPLE 12

To 100 parts of cellulose diacetate were added 140 parts of polyethyleneglycol and diglycerol, and the mixture was molten at 200° C. and spunthrough spinnerets for circular or finned hollow fibers. The plasticizerwas extracted by hot water. The hollow fibers were filled in shellsdiffering in the size and the in vitro performance was measured. Theobtained results are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________           Membrane                                                                      Thickness (μ) of         Occupancy                                         Fin-Free Portion                                                                       Number                                                                             S         CLU/                                                                              Ratio                                             in Wet State                                                                           of Fins                                                                            (m.sup.2)                                                                        UFR CLU                                                                              UFR (%)                                        __________________________________________________________________________    Comparative                                                                          20       0    0.8                                                                              6.0 150                                                                              25.0                                                                              55                                         Example 8                                                                     Example 20                                                                           20       3    "  4.5 162                                                                              36.0                                                                              50                                         Example 21                                                                           20       6    "  4.0 165                                                                              41.3                                                                              45                                         Example 22                                                                           20       8    "  3.8 166                                                                              43.7                                                                              42                                         Comparative                                                                          20       0    1.8                                                                              12.1                                                                              175                                                                              14.5                                                                              55                                         Example 9                                                                     Example 23                                                                           20       3    "  7.0 180                                                                              25.7                                                                              50                                         Example 24                                                                           20       6    "  6.4 183                                                                              28.6                                                                              45                                         Example 25                                                                           20       8    "  6.0 183                                                                              30.5                                                                              42                                         Comparative                                                                          25       0    1.2                                                                              6.5 168                                                                              25.8                                                                              55                                         Example 10                                                                    Example 26                                                                           25       3    "  5.8 175                                                                              30.2                                                                              50                                         Example 27                                                                           25       6    "  5.5 178                                                                              32.4                                                                              45                                         Example 28                                                                           25       8    "  5.3 180                                                                              34.0                                                                              42                                         Comparative                                                                          35       0    0.8                                                                              3.0 120                                                                              40.0                                                                              50                                         Example 11                                                                    Example 29                                                                           25       6    "  3.5 145                                                                              41.4                                                                              45                                         Comparative                                                                          55       0    0.8                                                                              2.5 113                                                                              45.2                                                                              50                                         Example 12                                                                    Example 30                                                                           40       6    0.8                                                                              3.5 135                                                                              42.2                                                                              45                                         __________________________________________________________________________

EXAMPLE 31

To 100 parts of cellulose diacetate were added 230 parts of polyethyleneglycol (having a molecualr weight of 200) and diglycerol, and themixture was molten at 190° C. and the melt was spun through a spinnerethaving an annular double slit for finned hollow fibers while introducingnitrogen gas into the core portion. In the obtained finned hollowfibers, the inner diameter was about 200 μ, the membrane thickness ofthe fin-free portion was about 25 μ, the fin height was about 35 μ, thefin root width was about 25 μ and the number of fins was 6.

The finned hollow fibers were bundled and supplied on a metal net havingan opening size of 5 mm, and on this metal net, the polyethylene glycoland diglycerol were extracted by hot water to obtain a crimped finnedhollow fiber bundle in which the amplitude was about 200 μ, thewavelength was about 15 mm and the minimum radius of curvature was about25 mm.

A blood dialyzer having a filling ratio of about 50% and an effectivemembrane area of about 1 m² was assembled by using this hollow fiberbundle, and the in vitro ultrafiltration performance and dialysisperformance were measured. Furthermore, the state of coagulation ofblood and the amount of the residual blood were checked. The obtainedresults are shown in Table 5.

EXAMPLES 32 AND 33

Finned hollow fibers before the crimping treatment, which were obtainedin the same manner as described in Example 31, were subjected to hotwater extraction on a metal net having an opening size of 3 mm or 10,mm, to obtain a hollow fiber bundle having an amplitude of about 80 μ,a wavelength of about 10 mm and a minimum curvature radius of about 15mm or a hollow fiber bundle having an amplitude of about 300 μ, awavelength of about 30 mm and a minimum curvature radius of about 30 mm.In the same manner as described in Example 31, blood analyzers wereassembled by using these hollow fiber bundles and the characteristicswere determined. The obtained results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                               UFR       Urea               Amount (ml)                               Example                                                                              (ml/m.sup.2 ·                                                                  Dialysance                                                                              Coagulation                                                                            of Residual                               No.    hr · mmHg)                                                                     (ml/min)  of Blood Blood                                     ______________________________________                                        31     4.3       175       not caused                                                                             0.13                                      32     4.0       178       not caused                                                                             0.15                                      33     4.7       173       not caused                                                                             0.10                                      ______________________________________                                    

EXAMPLES 34 AND 35

To 100 parts of cellulose diacetate was added parts of polyethyleneglycol (having a molecular weight of 300), and the mixture was molten at200° C. and spun through a spinneret having an annular double slitprovided with such notches and connecting portions as giving finnedhollow fibers having fin height and distance shown in Table 6. Then, thehollow fibers were saponified to convert cellulose diacetate tocellulose, followed by winding. Blood dialyzers having an effectivemembrane area of about 1 m² were assembled by using these hollow fibers.The configurations of the hollow fibers and the performancecharacteristics of the dialyzers were as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                         Example 34                                                                            Example 35                                           ______________________________________                                        Number of Fins      3         4                                               Maximum Fin Height (μ)                                                                         35        40                                              Minimum Fin Height (μ)                                                                         10        15                                              Maximum Distance (μ)                                                                          100       100                                              between Two Adjacent Fins                                                     Minimum Distance (μ)                                                                          350       400                                              between Two Adjacent Fins                                                     Occupancy Ratio (%) in                                                                            50        45                                              Dialyzer                                                                      Urea Dialysance of 173       175                                              Dialyzer                                                                      Tube Plate Leak Ratio (%)                                                                           1.2       0.8                                           ______________________________________                                    

EXAMPLES 36 THROUGH 45 AND COMPARATIVE EXAMPLES 13, 14 and 46

To 100 parts of cellulose diacetate was added polyethylene glycol(hereinafter referred to as "PEG") (having a molecular weight of 300) inan amount shown in Table 6, and the mixture was molten at 200 to 240° Cand extruded at a temperature shown in Table 7 at the extrusion openingthrough a nozzle shown in FIG. 6 (the inner diameter of the annulardouble slit was 1.6 mm, the outer diameter was 2 mm, the knotch widthwas 0.15 mm and the knotch height was 0.7 mm). The extrudate was cooledby air fed at a rate shown in Table 7 at room temperature, followed bywinding.

The melt viscosity of the polymer melt at each temperature wasseparately measured by a flow tester.

The distance between the solidification point and the top end of thenozzle was measured while clipping the fiber being spun by a clipper,and the point where attenuation terminates was regarded as thesolidification point. At a nozzle temperature of 190° C, the viscosityof the polymer melt was too high and yarn breakage was readily caused,and hence, winding could not be carried out smoothly.

Hollow fibers wound were saponified by an alkali to convert cellulosediacetate to cellulose and obtain finned hollow fibers as shown in FIG.6 (the thickness of the fin-free portion was about 20 to about 25 μ).Dialyzers were assembled by filling these hollow fibers in shells atsubstantially the same filling ratio. The in vitro dialysis performancewas determined. The obtained results are shown in Table 7.

In the hollow fibers obtained according to the present invention, theshape of the fins was sharp and the width of the root portion of thefins was narrow, and the dialyzer comprising the hollow fibers of thepresent invention had a high performance value. On the other hand, incase of the comparative hollow fibers outside the scope of the presentinvention, the shape of the fins was rounded and fins having asufficient height could not be formed.

                                      TABLE 7                                     __________________________________________________________________________                             Distance (cm)                                                Amount                                                                             Nozzle Viscosity                                                                          between Nozzle                                               (parts)                                                                            Temperature                                                                          (P)  and Solidifi-                                                                         Draft                                                                             Feed Rate (m/sec) Urea Clearance         No.     of PEG                                                                             (°C.)                                                                         at Nozzle                                                                          cation Point                                                                          Ratio                                                                             of Cooling Air                                                                          H (μ)                                                                          W (μ)                                                                          in                     __________________________________________________________________________                                                           Dialyzer               Example 36                                                                            50   200    2900 12      60  0.5       45  24  168                    Example 37                                                                            50   210    1500 15      60  0.5       42  30  168                    Example 38                                                                            50   220    980  26      60  0.5       37  35  165                    Example 39                                                                            50   225    550  29      60  0.5       26  42  162                    Example 40                                                                            50   230    450  38      60  0.5       20  50  160                    Example 41*.sup.1                                                                     50   210    1500 20      108 0.5       40  32  165                    Example 42                                                                            30   200    5000 10      60  0.5       40  28  167                    Example 43                                                                            120  220    200  35      60  0.5       28  40  160                    Example 44                                                                            50   225    550  25      60  1.0       36  35  164                    Example 45                                                                            50   225    550  18      60  2.0       40  30  166                    Comparative                                                                           50   225    550  43      60   0*.sup.3 18  50  150                    Example 46                                                                    Comparative                                                                           50   190    11000                                                                              --      60  0.5       --  --  --                     Example 13*.sup.2                                                             Comparative                                                                           50   240     80  45      60  0.5       15  50  147                    Example 14*.sup.3                                                             __________________________________________________________________________     Note                                                                          *.sup.1 spinneret having a large nozzle was used                              *.sup.2 yarn breakage was caused                                              *.sup.3 atmosphere was heated at about 50° C.                     

EXAMPLES 47 through 50 AND COMPARATIVE EXAMPLES 15 AND 16

In the same manner as described in Example 36, a polymer shown in Table8 was melt-spun and the spun fibers were drawn to render them finelyporous. Hollow fibers provided with fins having configurations shown inTable 8 were obtained.

                                      TABLE 8                                     __________________________________________________________________________                 Nozzle Viscosity                                                                          Distance (cm)                                                     Temperature                                                                          (P) at                                                                             between Nozzle                                                                            H  W                                            Polymer                                                                             (°C.)                                                                         Nozzle                                                                             and Solidification Point                                                                  (μ)                                                                           (μ                                 __________________________________________________________________________    Example 47                                                                           Poly- 200    560  16          38 28                                           ethylene                                                               Example 48                                                                           Poly- 210    450  28          30 30                                           ethylene                                                               Comparative                                                                          Poly- 220     85  45          15 54                                    Example 15                                                                           ethylene                                                               Example 49                                                                           Poly- 220    680  18          42 25                                           propylene                                                              Example 50                                                                           Poly- 230    510  30          32 32                                           propylene                                                              Comparative                                                                          Poly- 240     90  45          13 50                                    Example 16                                                                           propylene                                                              __________________________________________________________________________

EXAMPLE 51

To 100 parts of cellulose diacetate (having an average polymerizationdegree of 170) was added 50 parts of polyethylene glycol (having amolecular weight of 200), and the mixture was molten at 230° C and wasextruded through an annular double slit for finned hollow fibers whileintroducing nitrogen gas into the core portion. The spun fibers wereimmersed in hot water to dissolve out the polyethylene glycol. Then, thesaponification was carried out with an aqueous solution of sodiumhydroxide and the fibers were immersed in an aqueous solution containingglycerol at a concentration of 80% by weight and were then dried by hotair to obtain finned hollow fibers for the hemodialysis, which had aninner diameter of about 200 μ, a fin-free portion thickness of about 18to 30 μ and 6 fins having configurations shown in Table 8. The re-wetelongation, the tensile strength in the wet state, the flexural rigidityand the rupture pressure of the hollow fibers were as shown in Table 9.

The so-obtained hollow fibers were filled in a circular tube shell toassemble a blood dialyzer having an effective membrane area and fillingratio shown in Table 9. The in vitro ultrafiltration performance anddialysis performance and the amount of the residual blood were measured.The obtained results are shown in Table 9.

EXAMPLES 52 THROUGH 54

Spinning was carried out in the same manner as described in Example 51by using cellulose diacetate having an average degree of polymerizationof 160, 180 or 200 to obtain hollow fibers having 6 fins havingconfigurations shown in Table 9. Blood dialyzers were assembled in thesame manner as described in Example 51. The characteristics of thehollow fibers and blood dialyzers were measured. The obtained resultsare shown in Table 9.

COMPARATIVE EXAMPLES 17 AND 18

Hollow fibers and blood dialyzers were prepared in the same manner asdescribed in Example 52 by using cellulose diacetate having a degree ofpolymerization of 120 or 140. The characteristics of the hollow fibersand dialyzers were measured. The obtained results are shown in Table 9.

EXAMPLES 55 and 56 AND COMPARATIVE EXAMPLE 19

To 100 parts of cellulose diacetate (having an average degree ofpolymerization of 180) were added 135 parts in total of polyethyleneglycol and diethylene glycol, and the mixture was molten at 200° C andextruded through a spinneret having an annular double slit for finnedhollow fibers. The plasticizer was dissolved out by hot water. Thus,there were obtained hollow fibers in which the inner diameter was 205 μ,the thickness of the fin-free portion was 12 to 15 μ and the number offins was 6. Blood dialyzers were assembled by using these hollow fibers.The characteristics of the hollow fibers and dialyzers were measured.The obtained results are shown in Table 9.

                                      TABLE 9                                     __________________________________________________________________________                Example                                                                            Example                                                                            Example                                                                            Example                                                                            Comparative                                                                          Comparative                                                                          Example                                                                            Example                                                                            Comparative                       51   52   53   54   Example 17                                                                           Example 18                                                                           55   56   Example               __________________________________________________________________________                                                            19                    Fin Height (μ) (H)                                                                     26   20   28   35   13     13     10   8    6                     Fin Width (μ) (W)                                                                      20   25   20   18   30     28     15   12   15                    Re-wet Elongation (%)                                                                     1.5  1.8  1.2  0.8  3.5    3.2    0.5  0.6  0.8                   Wet Strength (g/de)                                                                       0.5   0.45                                                                               0.53                                                                               0.58                                                                               0.30   0.35  0.6  0.6  0.5                   Flexural Rigidity                                                                         250  220  250  280  180    170    400  340  350                   (wet) (mg)                                                                    Rupture Pressure                                                                          4.3  4.0  4.8  5.5  3.0    3.4    4.5  4.0  4.2                   (wet) (kg/cm.sup.2)                                                           Effective    1.10                                                                               1.15                                                                               1.03                                                                                0.96                                                                              1.20   1.20   1.20                                                                               1.20                                                                               1.20                 Membrane Area (m.sup.2)                                                       Occupancy Ratio (%)                                                                       46   48   43   40   50     50     45   45   50                    UFR (ml/mmHg · hr)                                                               4.8  5.0  4.5  4.0  6.0    5.4    6.5  7.0  6.5                   Dialysance (ml/min)                                                                       168  167  170  170  163    164    165  168  158                   Amount (ml) of                                                                            below 0.1                                                                          below 0.1                                                                          below 0.1                                                                          below 0.1                                                                          0.3    0.3    below 0.1                                                                          below                                                                              0.2                   Residual Blood                                                                Coagnlation of Blood                                                                      not  not  not  not  slight slight not  not  not caused                        caused                                                                             caused                                                                             caused                                                                             caused             caused                                                                             caused                     __________________________________________________________________________

We claim:
 1. A process for preparing, by melt-spinning, aselective-permeable hollow fiber for blood dialysis having a ureaclearance not less than 160 ml/min and having an outer diameter of about100 to 500 μ, a hollow portion piercing in the longitudinal directionand 1 to 10 fins extended in the longitudinal direction on a peripherythereof the fins having a height H to width W ratio H/W of at least 0.4,wherein a spinning liquid containing at least one cellulose esterpolymer having an average degree of polymerization of at least 150 andat least one additive for forming pores is extruded into a gas phasefrom a spinning nozzle for said hollow fiber having an annular doubleslit and 1 to 10 notches formed on an outer side of the double slit, theviscosity of the spinning liquid at said nozzle is 200 to 5,000 P, thedistance between the spinning nozzle's end face and the point where thehollow fiber extruded from the spinning nozzle is solidified in the gasphase is less than about 40 cm, and the solidified hollow fiber istreated so that it has selective-permeable properties.
 2. A processaccording to claim 1, wherein the viscosity of the spinning liquid is300 to 2,000 P.
 3. A process according to claim 2 wherein the viscosityof the spinning liquid is 500 to 1,500 P.
 4. A process according toclaim 1 wherein the solidified hollow fiber is treated by extraction toremove the additive.
 5. A process according to claim 1 wherein thesolidified hollow fiber is treated with alkali.
 6. A process accordingto claim 1, wherein a plasticizer of cellulose ester polymer iscontained in the spinning liquid.
 7. A method for preparing aperm-selective hollow fiber for blood dialysis having a urea clearancenot less than 160 ml/min having fins disposed along an outer surfacethereof the fins having a height H to width W ratio H/W of at least 0.4,pl comprising the steps of:preparing a spinning liquid comprised of acellulose ester polymer having an average degree of polymerization of atleast 150 and a pore forming additive; extruding said spinning liquidfrom a spinning nozzle into a gas phase through an annular double slitin an end face of said spinning nozzle including notches for formingsaid fins; maintaining extruding conditions so that the viscosity ofsaid spinning liquid at said spinning nozzle is between about 200 and5,000 P and solidifying conditions so that extruded spinning liquid'ssolidification point is less than about 40 cm from said double slit; andtreating the solidified hollow fiber so that it has selectivepermeability properties.